优化子查询选择每个组的最后一条记录

Question

我有这个查询,这是一个依赖查询,需要很长的执行时间

SELECT
  u.id,
  u.user_name,
  ifnull((select longitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
  ifnull((select latitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
  (select  created  from map  where user_id = 1  order by created desc   limit 1) as LatestTime
FROM users as u
WHERE id IN(SELECT
          user1_id FROM relation
        WHERE users.id = 1)
ORDER BY id;

我试过(依赖)这个查询

SELECT
  u.id,
  u.user_name,
  m.map_id,
  m.longitude,
  m.latitude,
  m.Date as created
FROM users as u
  left join (select
           map_id,
           longitude,
           latitude,
           user_id,
           max(created) as `Date`
         from map
         group by user_id) as m
    on m.user_id = u.id
WHERE id IN(SELECT
          user1_id FROM relation
        WHERE users.id = 1)
ORDER BY id;

问题是第一个查询依赖并且工作正常,但需要很长的执行时间.对于第二个查询,问题是它没有获取最新创建的时间.现在我想优化这个查询.主题是在子查询中我首先创建组,然后我试图获得每组的最后记录.这是表格结构.

users : id , user_name
map   : map_id ,  user_id ,longitude , latitude, created 
relations : id , user1_id , user2_id , relation

Answer 1

在需要性能的地方,SELECT子句中的子查询确实很痛苦,必须放逐:)

你可以重写这个部分:

SELECT
    u.id,
    u.user_name,
    ifnull((select longitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
    ifnull((select latitude from map where user_id = u.id order by map_id desc limit 1 ),0) as Longitude,
    (select  created  from map  where user_id = 1  order by created desc   limit 1) as LatestTime
FROM users as u

在:

SELECT
    u.id,
    u.user_name,
    COALESCE(m1.longitude, 0) as longitude,
    COALESCE(m1.latitude, 0) as latitude
FROM users u
LEFT JOIN map m1 ON m1.user_id = u.id
LEFT JOIN map m2 ON m2.user_id = m1.user_id AND m2.map_id > m1.map_id
WHERE m2.map_id IS NULL

我在这个答案中写了一个简短的查询结构解释.这是一个非常好的技巧,因为它更具可读性,无子查询和更高性能.

我还没有看过这个IN部分,但如果上面没有帮助的话.

Edit1:您可以提取创建的日期并使用MAX()代替.

SELECT
    u.id,
    u.user_name,
    COALESCE(m1.longitude, 0) as longitude,
    COALESCE(m1.latitude, 0) as latitude,
    created.LatestTime
FROM (SELECT MAX(created) FROM map WHERE user_id = 1) created
INNER JOIN users u ON TRUE
LEFT JOIN map m1 ON m1.user_id = u.id
LEFT JOIN map m2 ON m2.user_id = m1.user_id AND m2.map_id > m1.map_id
WHERE m2.map_id IS NULL